When Jupiter meets Saturn with the Schmidt telescope

On the 2020 winter solstice, Jupiter passed Saturn in a great conjunction in dusk. This apparent (line-of-sight only) encounter, easily visible to the naked eye as a bright “double star” in the evening sky, was caught with the smallest of the four main telescopes in Calar Alto: the 1-meter class Schmidt telescope.

This historical telescope (Großer Hamburger Schmidtspiegel) represents the culmination of the know-how of Bernhard Schmidt, a German optician who proposed, in 1930, a brand new and revolutionary concept for a wide-field but nearly perfect quality optics telescope.  Despite the construction of a “great Schmidt”, of about one meter in diameter, was planned to start in 1937 inside the famous Carl Zeiss factory in Jena, World War II delayed its final commissioning to 1951.

Initially installed at Hamburg university observatory, near the village of Bergedorf, this site soon appeared to be hardly usable for serious astronomical research, due to the poor weather and growing light pollution in the Hamburg countryside. In the late 1970s, astronomers from Hamburg and MPIA Heidelberg thus decided to move the precious optical tube to Calar Alto; in 1980, the great Schmidt was commissioned again with a new, English made mount adapted to the lower Andalusian latitude of its new observatory.

Under the crystal-clear skies of Calar Alto, the Schmidt telescope fruitfully observed huge and deep stellar fields for nearly two decades, using large photographic plates and films. Around 25x20 cm in size, these giant “analog” sensors located in the focal plane of the Schmidt could cover in a single shot an ample field of view – equivalent to about 150 full moons.

Nonetheless, by the late 1990s, with the omnipresence of digital detectors in professional observatories and the cease of production by the main photographic plate manufacturer (Kodak), the Schmidt, with its “analog” sensors, was no more competitive. By the year 2000, the Schmidt telescope was no longer used at the observatory, although its legacy value is ensured thanks to its digitized photographic archive.

After a 15-year-long break, still maintained in perfect shape by the Calar Alto technical staff, it was decided to perform a complete, home-made upgrade of the Schmidt: telescope control system, drives, focal plane… So as to install modern, albeit smaller, digital detectors (Charge Coupled Devices or CCDs) instead of the former, large photographic plates and films.

In addition, the renovation of the Schmidt was done with the idea of using the telescope remotely. Researchers collaborating with the Planetary Defence Office of the European Space Agency (ESA) have been indeed successfully using the Schmidt from the distance since 2015. Mostly to look for Near-Earth Objects (NEOs), some of them representing potentially hazard for our planet.

Various follow-up observations and recoveries were made in the last years with the Schmidt, which is able to detect faint objects (nearly a million times fainter than what human eyes can see) using, typically, one minute exposure times. The high sensitivity of the Schmidt telescope is due to the wide aperture -- an 80 cm aspherical corrector plate, made of a special UBK7 glass, in combination with an over-dimensioned 1.2-meter spherical mirror -- and fast focal ratio  -- f/3, similar to high-end telephoto lenses of current DSLR or mirrorless cameras--, as well as the high quantum efficiency of modern digital sensors like the CCD currently mounted, property of DEIMOS Space.

Observing the conjunction: One size does not fit all!

Despite the Schmidt telescope and its sensitive CCD detector not being designed for observing bright planets, nor for pointing at low elevation, Elisabeta Petrescu, a Young Graduate Trainee of ESA’s Planetary Defence Office, managed to observe the Jupiter-Saturn “great conjunction” in twilight around the winter solstice; the cold season officially started at 10:02 UTC on December 21, 2020.  The two biggest planets in our Solar System indeed apparently met during this very special evening; they were located low above the South-West horizon, separated by 6 arcminutes only (a fifth of the full moon apparent diameter) during the first winter dusk.

In Elisabeta’s words “It was a battle between the twilight sky and the low elevation of the planets, but even so, we managed to get an impressive amount of Saturn`s satellites with the exposure used. The images are taken when the planets were 17 degrees above the horizon, with a 0.1-second exposure. I am honored to use such a beautiful and powerful telescope”.

Because the Schmidt CCD camera was designed to be used for long exposures, the shutter cannot take exposures shorter than one tenth of a second: for such an aperture, short focal-ratio telescope, the giant planets will thus always appear heavily saturated in any image taken, except when seen through pretty thick clouds. The strong saturation provokes heavy “bleeding” of the electrons in the CCD sensor, which is viewed as elongated, vertical “white drops” in the image, above and below the two planets. Although far from being aesthetic, the image from the Schmidt is quite significant of how stunningly bright a naked-eye object appears in a professional telescope.

On the other hand, even the shortest attainable 0.1 s exposure with the Schmidt already shows the brightest satellites, or moons, of the two giant gaseous planets. Some of these rocky worlds around several solar system planets (and possibly, around other stars) may harbor underground water in liquid form. The main satellites are labeled in the image taken on December 21st by Elisabeta: half-a-dozen for Saturn, including the bright Titan, as well as the four Galilean Moons of Jupiter.

The animated GIF compares the unlabeled image of the closest conjunction with another one taken the day after, i.e. on December 22nd -- under thin cirrus, explaining the somewhat less saturated images. From this simple animation (both images are centered on Jupiter), one can easily see how the two giant planets slowly moved away in a bit less than 24 hours. In reality, Jupiter and Saturn were located more than 730 million of kilometers apart from each other, but they did appear very closely projected on the line-of-sight from Earth those evenings before Christmas 2020. Obviously, the main satellites also changed their position around their host planet.

animation time

During the winter of 1609-1610, Galileo Galilei already observed, with one of the first telescope (better say, spyglass), the “ballet” of Io, Europa, Ganymede and Callisto around Jupiter, from one night to the other. It was the first direct evidence that not every body was orbiting the Earth in the wrong geocentric-based doctrine taught at this time…

The last time Jupiter and Saturn were so close in the sky, while away enough from the Sun to be visible with an average sight, occurred nearly 800 years ago. Fortunately enough -- at least for the youngest amateur astronomers--, in 2080, a great conjunction between the two giant planets will occur again. The next generation of telescopes in Calar Alto will hopefully catch again the event, as a tribute to the venerable, first-ever designed, wide-field 1-meter class telescope.

Although planetary conjunctions are not at all relevant scientifically speaking, they remember us how time is passing and how spectacular the sky at night can be, at least in the remaining dark places on our little planet…

Season’s greetings from Calar Alto and best wishes for 2021 to all our users, at ESA and beyond!


Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on November 6th, 2018, by the Science, Technology and Innovation Policy Council

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